Organic halogenated compounds are obtained in relatively large amounts as by-products of various industrial processes. Representative - but not limitative - examples of such compounds are chloro- or bromo-aromatic compounds, such as polychlorinated and polybrominated biphenyls (PCBs and PBBs), polychloro heterocyclic compounds, such as p-hexachlorocyclohexane, and organic solvents such as chlorobenzene. These products are toxic and hazardous, and must be disposed of in an effective manner.
Disposal of PCBs by incineration is expensive, due to the thermal stability of these compounds and it is complicated because highly toxic substances, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin may be emitted during the process. Only a few specialized incinerators are licensed to handle such dangerous materials, and the facilities in which these processes are carried out are accused of causing environmental pollution [New Scientist, 14.10.89]. Because of these problems, many efforts have been made in the art to develop effective and safe processes for the chemical degradation of halogenated organic compounds, especially PCBs.
The Prior Art
Many processes have been provided in the art, including processes for the chemical treatment and reclamation of oils and liquids containing various quantities of halogenated hydrocarbons. Processes of this type can be divided into two main categories. The first type of process includes the reductive dehalogenation, wherein the organic substances are treated with hydrogen gas (e.g., U.S. No. 4,840,721, U.S. No. 4,818,368, EP 306,164 and EP 299,149), or with other hydrogen donating compounds such as alkali hydride (GB 2,189,804), hypophosphite (U.S. No. 4,618,686), sodium borohydride (U.S. No. 4,804,779). These processes present several severe drawbacks, because they usually involve either complicated hydrogenation processes using explosive gases at high temperatures and pressures, which must be performed in specially designed reactors, or they involve the use of special reagents which are unfavored in industry for economical and safety reasons. Furthermore, HC1 is produced in the process, which, as will be apparent to a skilled chemist, represents an added complication.
The second type of dehalogenation processes involves the reactions of metals, alkali earth metals, alkali metals, or compounds of these metals which are chemically capable of causing the degradation of a carbon-halogen bond, and which lead to the transformation of the organic halogen into an inorganic halogen bonded to the metal. Some examples of such processes are the use of metal or metals compounds such as tin, lead, aluminum, chloroaluminates, titanium, aluminum oxide, etc. (EP 277,858, EP 184,342 and U.S. No. 4,435,379). The most used compounds are alkali metals and alkali metal compounds such as sodium/sodium hydroxide (U.S. No. 4,755,628, CA 1,185,265 and EP 99,951), sodium naphthalene, sodium polyethylene glycol (EP 140,999 and EP 60,089), sodium carbonate, bicarbonate, alcoholates, etc. (U.S. No. 4,631,183 and EP 306,398).
Processes of this type also present considerable drawbacks. For the less reactive metals, dehalogenation usually involves high temperatures, in the order of 500.degree.-1000.degree. C., which are needed for the cleavage of the stable carbon-chlorine bond, and for the purpose of bringing the metal into contact with the organic compound in the form of molten salt, fine dispersion, etc.
Active metallic compounds, on the other hand, may react at lower temperatures, in the order of 300.degree.-600.degree. C. However, a large excess of expensive reagents are needed, and the process involves separation and purification steps which render it both complicated and expensive.
Metallic compounds capable of inducing the dehalogenation at low temperatures are very reactive, and therefore their handling and use are limited by the need for rigorous anhydrous conditions and inert atmosphere, which are required to avoid the danger of uncontrolled exothermic decomposition of these compounds. These processes, therefore, are highly hazardous and expensive.
It is therefore clear that it would be highly desirable to provide a process for the dehalogenation of waste organic compounds which is both simple and inexpensive.